WEDNESDAY, Jan. 15, 2014 (HealthDay News) -- Excess weight might increase the risk of premature death among people with type 2 diabetes, according to a large new study that could upend earlier research. And the heavier a person is, the greater their risk of dying early.

These latest findings call into question previous studies that found an "obesity paradox." Those results suggested that being overweight might actually provide some protection from dying.

"We wanted to address the so-called obesity paradox," said the study's lead author, Deirdre Tobias, a research fellow at the Harvard School of Public Health. "It seemed implausible."

"In our much larger data set, I think this data supports the conclusion that the obesity paradox is a myth for those with type 2 diabetes," Tobias said.

Being overweight or obese is a well-established risk factor for premature death from conditions such as heart disease or cancer, according to background information in the study. But with some chronic conditions, such as heart failure, kidney failure and type 2 diabetes, some studies have suggested that people who are overweight or obese have a lowered risk of death.

Previous research, however, often has been done with small sample sizes. These studies haven't been able to control optimally for smoking and for other pre-existing diseases, according to the current study's authors.

People with type 2 diabetes don't make enough insulin or don't use the hormone properly to convert the food they eat into energy. As a result, their blood-sugar levels are too high.

The current study -- published Jan. 16 in the New England Journal of Medicine -- included more than 11,000 people with type 2 diabetes from the well-known Nurses' Health Study and the Health Professionals Follow-up Study. On average, their diabetes diagnoses occurred in their early 60s.

During nearly 16 years of follow-up, almost 3,100 people died, the researchers said. When they looked at the whole group, it appeared that being overweight or even slightly obese was less of a risk factor for dying than being normal weight. For example, someone with a body-mass index (BMI) -- a rough estimate of body fat based on height and weight -- between 27.5 and 29.9 was less likely to die than someone with a BMI between 18.5 and 22.4.

A normal BMI is between 18.5 and 24.9, according to the U.S. Centers for Disease Control and Prevention. Between 25 and 29.9 is overweight, and over 30 is obese.

But when Tobias and her colleagues separated the data by smoking status, the obesity paradox disappeared for people who never smoked.

The researchers also looked at the data to see the relationship between BMI just before a diabetes diagnosis and deaths due to heart disease, cancer and other causes. They found that the higher the BMI, the greater the risk of dying from cardiovascular disease in those who never smoked.

"Smokers tend to be leaner, and that may attenuate the risk of obesity or even make it look protective," Tobias said. "But when we stratified the data by smoking status, we saw the relationship is truly linear, with mortality risk going up with BMI."

The bottom line: Being overweight or obese doesn't confer a survival benefit on people with diabetes, Tobias said. "Weight management still remains an important component of type 2 diabetes management," she said.

Dr. Joel Zonszein, director of the clinical diabetes center at Montefiore Medical Center in New York City, agreed that the obesity paradox is a myth.

He said one of the reasons for so much conflicting information in studies is that using BMI alone as an indicator of weight status is insufficient.

"BMI doesn't tell us how much bad fat a person has," Zonszein said. "People have good fat and bad fat. It's more important to know about the bad fat. You have to look at BMI, along with other cardiovascular disease risk factors [such as high blood pressure and high cholesterol]. That would be much more reliable."

Based on current trends, the CDC estimates that by 2050 one in five Americans will have diabetes. Most people with diabetes have type 2, which is associated with being overweight and inactive.